Planning on brewing a DIPA this weekend. I haven't quite pieced together the recipe yet, but would it be possible to separately mash a small amount of some dark malts, then chuck these in with the leftover pale, crystal and pilsner and make a northern English Mild with the later runnings? Would there be enough sugar left to make something about 4% ABV?

I wouldn't try. The risk of extracting tannins is too great. The beer you end up with will be very small anyways (I'm guessing low-1.020's at best) and you would need a lot of extract to bring it up to snuff.

If you were making a humongous beer, then maybe you can get away with it, but definitely not with an IIPA.

yes it would work if you would collect sparges for 2nd beer, i can give you an example of my SG from last brew session last week
18lbs of grain mashed with 5.2gal of water i got 3gal back with 1.092 SG
then i sparges 2x with 3gal each, from 1st sparge i got 1.047SG and from 2nd sparge 1.028 hope this help

It can be made to work, but you should plan for it. Use your software or spreadsheet or pencil and paper to calculate what you will end up with before you start. Partigyling is not magic, but it is awesome.

What is the OG of the DIPA? What efficiency do you normally get on your system doing a single batch sparge?

By the way, if you have already bought the grains and plan on doing 2 sparges for the DIPA, then the die is cast. Likely you won't have enough left to do anything else--and the scary tannin monster will get you, too, if you try.

Unless you will only be running 2 pints or so through each pound of grain for your IPA--and thus assuming a low efficiency. A low efficiency means there's something good left to get.

You can do it by upping your grain bill based on your efficency. Take your first runnings and dilute to the proper pre boil amount and gravity. Use the rest, plus capping off with some grain, for your mild.